Allow blocks to define that they are only selectable when item can be placed on it

[DellieDelta]

To remove the .fluid case in item selection, preparation for #627

Adds the item field .placementTags and the block selectionRule .placeable

It's a simple version for now (selection happens if a placementTag overlaps with a block tag), but it can be changed in the future to some kind of condition checking. Like replacing it with .placementConstraints (like tagOverlap, onTop, onBottom), which can work similarly to .selectionRule. To get closer to fixing #2491

I'll also make it possible separately to define multiple selectionRules per block in #2987, to make both the selectionRules .toolEffective and .placeable possible on the same block, in preparation for adding buckets

I couldn't pick block as parameter name for isPlaceableOn() because block() is already defined. So I picked targetedBlock as parameter name instead